Weed Control in Corn:
Helping the Crop Compete
By Greg Stewart, OMAFRA Corn Specialist

1. Eliminate weeds immediately before planting (by chemical burn down or field cultivation, depending on the tillage system, to ensure weeds are not ahead of the crop).
2. Provide seedbed conditions which promote rapid and uniform emergence of the crop.
3. Band fertilizers in close proximity to the crop, to reduce weed access to nutrients.
4. Plant crops in row widths and densities which promote crop canopy development.

Narrowing rows and increasing populations has generally been considered a tool to enhance crop competitiveness in wheat and soybeans. Recently however, it has been discussed more and more as an option for corn producers, as narrow row corn with higher populations has increased in profile. Work at the University of Guelph under the direction of Prof. Clarence Swanton has examined the potential impact of these cropping system modifications on weed control in corn. The first of these studies examined two row widths (15 and 30 inches), two population densities (28,000 and 40,000 ppa) and three levels of weed control (weed free for the full season, weed free only until the fourth leaf stage of the corn crop, and weed free until fourth leaf stage and then weeds removed only in the inter-row zones). In these studies all weeds were controlled by hand.

Reducing Weed Growth
On the two experimental treatments where weeds were allowed to grow after the 4th leaf stage of corn growth, weeds were reduced by narrowing rows and by increasing populations (Table 1). Weed growth was measured at the twelfth- to fourteenth-leaf stage of development to determine the effect of row width and density on weed biomass.

Corn planted in either narrow rows or high densities (i.e., 15-inch rows or 40,000 ppa) appeared to out-compete weeds better than in more conventional corn cropping patterns (30-inch rows or 28,000 ppa). Swanton and his researchers claim this competitiveness comes principally in the form of superior sunlight interception by the corn crop. Table 2 illustrates the amount of sunlight that actually reached the soil surface under the various experimental treatments and shows the significant advantage to both narrower rows and higher densities in terms of sunlight interception. Across the three years of this study, narrow rows outyielded conventional row widths by 10 to 15 per cent when averaged over the three levels of weed control. This enhanced yield was at least partially credited to superior crop competitiveness with weeds when grown in narrow rows. However, high populations did not result in an improvement in yields relative to the lower populations. Raising the population to 40,000 ppa may have gone beyond the range where some yield gains were realized by higher populations.

Reducing Herbicide Requirements
Research along these same lines was conducted by Dr. Al Hamill, Agriculture and Agri-Food Canada, Harrow and Peter Sikkema, University of Guelph, Ridgetown and their research teams. These studies conducted at Centralia, Harrow and Ridgetown in both 1996 and 1997 evaluated the potential for high-population, narrow-row corn to maintain adequate weed control, while using reduced rates of post-emergent herbicide.

They planted corn in 15-, 20- and 30-inch rows at populations of 28,000 and 36,000 ppa, then imposed herbicide control with post-emergent applications of Marksman and Ultim at the full, recommended rate and at two substantially reduced rates. Interestingly, they achieved effective weed control across all plots with all of the herbicide approaches, either full or reduced rates.

This better-than-expected weed control masked any opportunity to determine the impact of row width and planting density on weed growth. Their results, however, indicated no advantage to corn yields from planting in narrower rows or at high densities. Sikkema generally believes the ability to use less herbicide in corn is often unrelated to row width and population and more related to weed pressure and timing of weed emergence within the corn crop.

Herbicide-Resistant Crops
Combining herbicide-resistant corn hybrids into these narrow row scenarios is also an area worth considering. Here, the argument is that enhanced crop competition may facilitate the use of herbicides with no residual weed control, or perhaps eliminate the need for a second in-crop application of the herbicide. Swanton has been working on this concept in experiments with Liberty Link corn planted in 15- and 30-inch rows at 28,000 and 40,000 plants per acre and controlled weeds with one or two applications of Liberty herbicide or with a more conventional Dual and Banvel treatment. Results from this experiment completed at Woodstock in 1996, 1997 and 1998 are outlined in Table 3 (low population data only). In these studies, yields were somewhat enhanced by narrow rows and the planting system may have provided opportunities for greater weed suppression within the various herbicide options.

Conclusions
1) Producers should not switch to narrow-row corn thinking yields will be consistently higher than in conventional planting patterns, nor should they think narrow-row corn will automatically allow for greater reductions in herbicide use than they could achieve in normal row widths.
2) Incentives for switching to narrow-row corn continue to come from a systems approach to cropping (i.e., one planter for beans and corn, more efficient use of no-till equipment, some enhancement of yields, etc.). Better crop competition and more efficient options for weed control may be another piece of this narrow-row puzzle.
3) Even if narrow rows or high populations are not a fit in your operation, producers should keep in mind the underlying importance of crop competitiveness in any weed control system. Get crops off to a fast start and focus on management decisions that favour the crop – such as good crop rotations, fertilizer banding, and precise timing of post-emergent herbicides – to ensure for your corn crop has every opportunity to out-compete the weeds.

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